1. Technical field
This invention relates to cable television systems and, more particularly, to a method and apparatus for transmitting data over a cable television channel susceptible to interference noise, the data being transmitted over a plurality of selectable data channels having carrier frequencies which are not harmonically related and which are located within a television bandwidth channel reserved for upstream transmission from a CATV subscriber to a headend control location. In accordance with the frequency selection method and apparatus of the present invention, the upstream transmit frequencies are automatically set on a periodic basis.
2. Description of the Prior Art
The development of cable television systems has reached the stage where not only is the provision of two way information flow desirable but is practically required by the implementation of new services. For example, in the implementation of impulse pay-per-view service where the subscriber may impulsively select an event for viewing and assume a charge, at least one data channel such as a telephone communication channel or an RF channel is required in an upstream (reverse) direction from a cable television subscriber to a cable television headend to report service usage data. Other uses for a return path include power meter reading, alarm services, subscriber polling and voting, collecting subscriber viewing statistics, and home shopping. While not every cable television system operator provides for two way transmission, manufacturers of cable television equipment have tended to provide for upstream transmission in the direction from the subscriber toward the headend. Practically all such manufacturers provide so-called split or two way systems having a spectrum of frequencies for upstream transmission which at least includes a band from 5 to 30 megahertz. This band of interest comprises cable television channel T7 (5.75-11.75 megahertz), T 8 (11.75-17.75 megahertz), T9 (17.75-23.75 megahertz) and T10 (23.75-29.75 megahertz). These return path channels, each having television signal bandwidth, may be used, for example, for video conferencing. Whether a so-called "sub-split", "mid-split" or "high-split" system is applied for two way transmission by a headend operator, all three types of split transmission systems typically involve an upstream transmission in the 5-30 megahertz band of interest.
An article entitled "Two-Way Cable Plant Characteristics" by Richard Citta and Dennis Mutzbaugh published in the 1984 National Cable Television Association conference papers demonstrates the results of an examination of typical cable television (CATV) return plants. Five major characteristics in the 5-30 megahertz upstream band were analyzed. These include white noise and the funneling effect; ingress or unwanted external signals; common mode distortion resulting from defective distribution apparatus; impulse noise from power line interference and other influences; and amplifier non-linearities.
White noise and Gaussian noise are terms often used to describe random noise characteristics. White noise describes a uniform distribution of noise power versus frequency, i.e., a constant power spectral density in the band of interest, here, 5-30 megahertz. Components of random noise include thermal noise related to temperature, shot noise created by active devices, and l/f or low frequency noise which decreases with increased frequency. The term noise floor is used to describe the constant power level of such white noise across the band of interest.
This noise is carried through each return distribution amplifier which adds its own noise and is bridged to the noise from all branches to a line to the headend. This addition of noise from each branch of a distribution tree in a direction toward a headend is known as noise funneling or the funneling effect. The constant noise floor power level defines a noise level which a data carrier power level should exceed.
The present invention is especially concerned with interference noise which causes peaks in the noise spectral density distribution in the band of interest. Interference noise destroys effective data transmission when known data transmission coding techniques such as frequency or phase shift keying are practiced over a single data transmission channel. In particular, interference noise especially relates to the four characteristics of return plant introduced above: ingress, common mode distortion, impulse noise and amplifier non-linearities.
Ingress is unwanted intended external signals entering the cable plant at weak points in the cable such as shield discontinuities, improper grounding and bonding of cable sheaths, and faulty connectors. At these weak points, radio frequency carriers may enter caused by broadcasts in, for example, the local AM band, citizen's band, ham operator band, or local or international shortwave band. Consequently, interference noise peaks at particular carrier frequencies may be seen in noise spectral density measurements taken on cable distribution plant susceptible to ingress.
Common mode distortion is the result of non-linearities in the cable plant caused by connector corrosion creating point contact diodes. The effect of these diodes in the return plant is that difference products of driving signals consistently appear as noise power peaks at multiples of 6 megahertz, i.e., 6, 12, 18, 24 and 30 megahertz in the band of interest.
Impulse noise is defined as noise consisting of impulses of high power level and short duration. Corona and gap impulse noise are created by power line discharge. Temperature and humidity are especially influential in determining the degree of corona noise, while gap noise is a direct result of a power system fault, for example, a bad or cracked insulator. The resultant impulse noise spectrum can extend into the tens of megahertz with a sin x/x distribution.
Amplifier nonlinearities or oscillations relate to pulse regenerative oscillations caused by marginally stable or improperly terminated amplifiers. The result is a comb of frequency peaks within the return plant band whose spacing is related to the distance between the mistermination and the amplifier.
From examining typical cable distribution plants, Citta et al. concluded that "holes" exist in valleys between peaks in the noise spectrum they plotted between 0 and 30 megahertz. They proposed that these valleys may be used to advantage by carefully choosing return carriers "slotted" in these valleys.
In follow-up articles published at the 1987 National Cable Television Conference and in U.S. Pat. No. 4,586,078 Citta et al., conclude that a 45 kilobit data signal may be alternately transmitted by a coherent phase shift keying (CPSK) technique over carriers at 5.5 megahertz and 11.0 megahertz or in the vicinity of the T7 and T8 cable television channels respectively. A switch at the subscriber terminal alternately selects the 5.5 MHz carrier or the harmonically related 11 MHz carrier for transmission. This form of alternating carrier transmission of messages is continued until the data is successfully received. In other words, alternating transmission on the two carriers occurs until an acknowledgment signal indicating successful receipt of a message is received at a terminal. While the choice of these carrier frequencies is claimed to avoid the noise distribution peaks caused by interference noise, there is considerable concern that such a modulated phase shift keyed data stream will run into noise peaks in cable television distribution network outside of the investigations of Citta et al. Referring to FIG. 2 republished here from U.S. allowed application Ser. No. 07/188,478 filed Apr. 29, 1988, transmission at 5.5 MHz should be practically impossible. Noise peaks have been known to appear and disappear based on time-of-day, season, and other considerations.
Other return path or upstream data transmission schemes have been tried. These schemes include, for example, the telephone system, described as "ubiguitous" by Citta et al. In other words, the return data path to a cable television headend is not provided over the cable television distribution plant at all. The serving cable is intentionally avoided either because of the interference noise problem in split system or because the system is a one way downstream system. Instead, the subscriber's telephone line is used for data transmission. In this instance, however, there is concern that local telephone data tariffs may require the payment of the line conditioning surcharges if the telephone line to a subscriber's home is used for data transmission in addition to normal "plain old" telephone service. Furthermore, the telephone line is only available when the subscriber is not using it, requiring an unscheduled or periodic data flow.
Another know return data transmission scheme involves the application of a separate data channel at a carrier frequency that avoids the troublesome 5-30 megahertz band. This scheme, of avoiding the noisy 5-30 megahertz band, is only possible in midsplit and high split systems.
So-called spread spectrum transmission of data is a technology which evolved for military requirements from the need to communicate with underwater submarines in a secure manner. Spread spectrum derives its name from spreading a data signal having an comparatively narrow bandwidth over a much larger spectrum than would be normally required for transmitting the narrow band data signal.
More recently the security advantages provided by spread spectrum transmission have been disregarded in favor of its capability of application in an environment of interference. For example, communications systems operating over a power line where impulse noise levels due to the power line are high have been attempted in the past but found to be only marginally acceptable, for example, power line plug-in intercom systems commercially available from Tandy Radio Shack. The Japanese N.E.C. Home Electronics Group, however, has demonstrated a spread spectrum home bus operating at 9600 baud over an AC line in a home that is practical up to distances of 200 meters of power line. The NEC system has been characterized as the missing link between a coaxial cable (for example, a cable television cable) and an AC power line common to the majority of homes.
U.S. Pat. No. 4,635,274 to Kabota et al. describes a bidirectional digital signal communication system in which spread spectrum transmission is applied for upstream data transmission in a cable television system. Such technology is very expensive, however, when compared with telephone data return.
Consequently, despite the development of spread spectrum and other RF data return, the requirement remains in the cable television art for an upstream data transmission having high data throughput from a plurality of subscriber premises to a cable television headend utilizing the cable television distribution plant and which is relatively impervious to interference noise.
The concept of Impulse Pay Per View (IPPV) is well understood in the art, but is described briefly here for completeness. Essentially it is a sales method by which a pay (cable) television subscriber may purchase specific program events on an individual basis. Furthermore, the purchase may be contracted on an "impulse" basis solely by interacting with the subscriber's in-home set-top terminal (STT). Although it is not a requirement that the event being purchased be "in progress", it is a requirement that the system support the purchase of events that are in progress. The purchase must be handled in a manner that does not incur any appreciable delay in the subscriber's ability to view the event immediately (i.e. instant gratification).
Although several techniques of implementing the above sales method exist, all techniques have common requirements. Some part of the system must make a decision whether or not to allow the purchase and subsequent viewing of the event. If allowed, the purchase of the specific event must be recorded and reported to what is typically known as the "billing system" so that the program vendor eventually receives revenue from the transaction.
To accomplish purchased event reporting, a so-called "store and forward" technique is used. In the store and forward method, the set-top terminal assumes that if the subscriber is pre-enabled for IPPV capability, then an event purchase is allowed. When the subscriber performs the necessary steps to purchase an event, the set-top terminal allows the event to be viewed (typically by de-scrambling a video signal on a particular channel) and records the purchase of the event. The record is typically stored in a secure, nonvolatile memory, as it represents revenue to the program vendor.
Obviously, in order to realize the revenue, the vendor's billing system must obtain the purchase record data stored in all of the subscribers' set-top terminals in a timely manner. To accomplish this, the system control computer (hereafter called the system manager) periodically requests that the set-top terminals return the IPPV purchase data stored in memory. When the system manager receives the data from a set-top terminal, it typically then acknowledges the receipt to the terminal (i.e., as does Citta et al.) and the data is cleared from memory to make room for additional purchase data. The system manager then forwards this data to the billing system, and the IPPV purchase cycle is completed.
While IPPV return data considerations are important to the determination of an RF data return technique, such IPPV return data considerations are not the only consideration, but admittedly are the most critical because of the high data throughput requirements. Other requirements such as using the return data path for subscriber polling, burglar alarm, meter reading, home shopping, energy management and the like are additive to the data throughput requirements of IPPV service.
Consequently, there remains a requirement in the art for RF data return apparatus having high data throughput to the degree of supporting a full range of services including IPPV service.